An intake valve for introducing mixed gas of fuel and air into a cylinder and an exhaust valve for discharging combustion gas outside the cylinder are used in an engine. Among these, since the exhaust valve is exposed to combustion gas having high temperature, a material having high temperature characteristics (for example, high temperature hardness, fatigue properties, high temperature strength, wear resistance and oxidation resistance) is used in the exhaust valve. Ni-based superalloy (for example, NCF751), austenitic heat-resistant steel (for example, SUH35) and the like are known as a material for exhaust valves.
Ni-based superalloy is a material in which γ′ phase has been precipitated by aging treatment, thereby enhancing strength at high temperature and wear resistance. The Ni-based superalloy is expensive, but has extremely high heat resistance. For this reason, a valve using this is mainly used in high output engines that are exposed to a temperature of 800° C. or higher.
On the other hand, the austenitic heat-resistant steel is a material in which M23C6 type carbide has been precipitated, thereby enhancing strength at high temperature and wear resistance. The austenitic heat-resistant steel is poor in high temperature characteristics as compared with the Ni-based superalloy, but is inexpensive. For this reason, a valve using this is mainly used in engines that do not require high heat resistance.
Various proposals have been conventionally made for materials suitable for such an exhaust valve.
For example, Patent Document 1 discloses a heat-resistant alloy for exhaust valves, containing, by weight %, C: 0.01 to 0.2%, Si: 1% or less, Mn: 1% or less, Ni: 30 to 62%, Cr: 13 to 20%, W: 0.01 to 3.0%, Al: 0.7% or more and less than 1.6%, Ti: 1.5 to 3.0%, B: 0.001 to 0.01%, P: 0.02% or less, and S: 0.01% or less, with the balance being Fe and unavoidable impurities.
Moreover, Patent Document 2 discloses an Fe—Cr—Ni heat-resistant alloy containing, by weight %, C: 0.01 to 0.10%, Si: 2% or less, Mn: 2% or less, Cr: 14 to 18%, Nb+Ta: 0.5 to 1.5%, Ti: 2.0 to 3.0%, Al: 0.8 to 1.5%, Ni: 30 to 35%, B: 0.001 to 0.01%, Cu: 0.5% or less, P: 0.02% or less, S: 0.01% or less, O: 0.01% or less, and N: 0.01% or less, with the balance being Fe and unavoidable impurities, and the alloy having a given component balance.
Furthermore, Patent Document 3 discloses a method for producing an automobile engine valve, comprising subjecting an Fe-based heat-resistant steel having a composition of Fe—0.53% C—0.2% Si—9.2% Mn—3.9% Ni—21.5% Cr—0.43% N to solution heat treatment at from 1,100 to 1,180° C., forging a valve head part at from 700 to 1,000° C., and subjecting to aging treatment.
This Patent Document describes that when the Fe-based heat-resistant steel having a given composition is subjected to solution heat treatment, forging and aging treatment under given conditions, a valve face part can be made to have hardness of HV 400 or more.
By recent sudden rise in raw material cost, production cost of an exhaust valve is greatly influenced by the fluctuation in raw material cost. In particular, because an Ni-based superalloy has large Ni content, raw material cost and production cost of an exhaust valve made of an Ni-based superalloy greatly receive the influence of Ni price. For this reason, a material in which an amount of Ni is further reduced and fluctuation width of raw material cost is decreased is desired. However, in an Ni-based superalloy, Ni is a forming element of γ′ phase that is a strengthening phase. Therefore, further reduction in the amount of Ni makes high strengthening utilizing γ′ phase difficult.
On the other hand, a carbide precipitation type austenitic heat-resistant steel is difficult to receive the influence of Ni price, but has a problem that high temperature characteristics are poor as compared with a γ′ phase precipitation type Ni-based superalloy. To solve this problem, a material obtained by highly strengthening SUH35 (for example, overseas standard LV21-43 steel (SUH35+1W, 2Nb)) is known. However, the LV21-43 steel still has the problems such that structure is difficult to control and hot workability is poor.